Finned tube heat exchanger and method of making same



United States Patent lnventor Charles S. Shriver Oreland, Pa.

Appl. No. 809,744

Filed Mar. 24, 1969 Patented Dec. 29, 1970 Assignee Orbit Manufacturing Company, Inc.

Perkasie, Pa. a corporation of Pennsylvania FINNED TUBE HEAT EXCHANGER AND METHOD OF MAKING SAME 3,396,458 8/1968 Mengetal. 2l9/366X Primary Examiner-Robert A. O'Leary Assistant Examiner-Theophil W. Streule Attorney-Howson and Howson ABSTRACT: A heat exchanger is disclosed wherein each of a plurality of platelike fins is mounted on an elongated tube by a series of interference fits between each fin and the tube. The interference fits are provided by forming a noncircular aperture in the fins, fonning a slightly smaller noncircular crosssectional shape on the tube, aligning the tube with the apertures, and displacing the tube longitudinally therethrough, and, after the fins are properly positioned on the tube, locking them in place by effecting relative rotation through a portion of a turn to effect an interference fit therebetween. As an additional feature, a novel clip is provided for mounting the fins and tube in a baseboard-type housing to permit the finned tubing to undergo normal thermal expansion and contraction with a minimum of noise while facilitating access thereto by also removably mounting its front cover.

PATENTED DECZQ [97B SHEET 1 [IF 2 CHARLES S. SHRIVER ATTYS.

INVENTORZ PATENTEU 050291970 SHEET 2 [IF 2 INVENTOR. if 54 CHARLES s. SHRIVER Mwf-W- ATTYB.

FINNED TUBE HEAT EXCHANGER AND METHOD OF MAKING SAME The present invention relates to heat exchangers of the finned-tube type, and more particularly, the present invention relates to finned tubing for use in baseboard heaters and to a method for affixing the fins onto the tubing.

At present, it is customary-to affix fins onto an elongated tube by providing a skirt around an aperture in each fin and crimping the skirt against the tube after the fins are properly positioned thereon. This method is not'entirely satisfactory because of the tendency for the skirts to expand radially out- .ward and possibly out of contact with the tube after the crimping force is removed therefrom. The rate of conductive heat transfer between the fins and the tube is thereby impaired, resulting in an inefircient heat exchanger. Accordingly, longer lengths of finned tubing are required to dissipate a given quantity of heat than would be required if the fins were secured to the tube in good conductive heat-transfer relationship. Other methods are also available for mounting fins on tubing. For example.the fins may be welded orf soldered onto the tubing; however, these are labor-consuming and expensive processes.

Finned tubing undergoes longitudinal expansion and contraction in response to the application of heat thereto and the removal of heat therefrom. When mounted in a baseboardtype housing, this expansion and contraction may generate'undesirable noise. Heretofore, simple means has not been provided for mounting finned tubing in a baseboard housing while permitting relatively silent operation. Furthermore, there is not presently available means which has the dual function of mounting the finned tubing in the housing while removably securing the front cover of the housing thereto to permit the cover to be readily removed for cleaning purposes or to permit the finned tubing to be removed and replaced.

With the foregoing in mind, it is a primary object of the present invention to provide novelfinned tubing which is effcient in operation. I

It is another object of the present invention to provide a simple method for making finned tubing wherein the fins are secured to the tubing in positive, conductive heat-transfer relationship. 7

It is a further object of the present invention to provide a method for making finned tubing in an economical manner.

As a further object, the present invention provides improved means for mounting finned tubing in a baseboard-type housing to provide relatively quiet operation and to facilitate access to its interior.

More specifically, in the present invention each of a plurality of platelike fins is secured in spaced-apart relationship on the length of an elongated tube by a series of interference fits provided between each fin and the tube. The interference fits are effected in the present invention'by forming a noncircular aperture in each fin and forming an identical but slightly smaller noncircular cross-sectional shape on the tubing. The tubingis then properly aligned with the apertures in the fins, and the tubing is displaced longitudinally therethrough. After the fins are located on the tubing, they are locked in place by rotating-the fins through portion of a turn relative to the tubing or vice versa, one of the elements being deformed to I thereby produce an interference fit between the fins and the tubing. In this manner, positive engagement is provided at spaced intervals betweeneach fin and the periphery of the tubing to facilitate conductive'heat transfer therebetweens When the finned tubing is disposed in a baseboard-heater housing; relatively quite operation is obtained by use of a series of novel clips which mount the tubing in the housing and which also removably secure the front cover of the housing thereto, one of the clips clamping the tubing centrally in the housing while permitting the ends to move longitudinally.

These and other'objects, features and advantages should becomeapparent from the following description when taken in conjunction with the accompanying drawings in which:

FIG. I is a perspective view of an electric baseboard heater embodying the present-invention and having portions of the front cover of its housing broken away to illustrate construction details in the interior thereof;

- FIG. 2 is a transverse sectional view taken alonglii'le 2-2 of FIG. 1 and illustrating a clip-mounting finned tubing and the front cover onupright webs which project forwardly from the rear wall of the housing; t

FIG. is an enlarged perspective view of one of the clips for mounting the tubing in the housing and for mounting the front cover thereto;

FIG. 5 is aschematic diagram illustrating a length of round tubing passing through a series of rollers which form the tubing into a substantially square cross-sectional configuration;

FIGS. 50 and 5b are sectional views taken along lines 50-54 and -5b-5b respectively to illustrate the cross, section of the tubing before and after passing through the rollers FIG. 6 is an end elevational view of FIGaS FIG. 7 is a perspective view of a platelikefin of the present invention with a portion broken away to illustrate the shape of its aperture and a skirt extending therearound;

FIG. 8 is a schematic diagram of a fixture which aligns the apertures in aseries of fins while the shaped tubingis displaced rightward therethrough;

FIG. 9 is a sectional view taken along line 9-9 of FIG. 8 to il-' lustrate the alignment between the tubing and the aperture in a finduring insertion of the tubing through the apertures;

FIG. 10 is an enlarged fragmentary view of'FlG. 9 illustrating a clearance between the tubing and the aperture in the fin;

FIG. 11 is a sectional view taken along line 9-9 of FIG. 8'

, after the fins have been rotated through an angle with respect 7 illustrating an interference fit between the tubing and the fin after the fins are rotated. I

. Referring now to the drawings, there is illustrated in FIG. f an electric baseboard-heater housing 10 with a length of finned tubing 11 mounted horizontally therein. As may be seen in FIG. 2, the heater housing 10 has an upright rear'wall l2 anda-front wall or cover 13 spaced therefrom to define a passage or airspace 14 for permitting air to fiow upwardly therebetween. The flow of air in the housing'is directed by for- 'wardly projectingdeflectors l5 and l6extending respectively along the upper and lower margins of the housing 10. In addi tion, a baffle 17 is mounted in the housing adjacent'theupper' deflector 15 to provide additional control of the air flow. In the present instance, the upper deflector 15 is formed integral with the rear wall 12, being spot-welded or riveted ther'etbizit 18 to provide a rigid structure. Similarly, the lower deflector 16 is integral with the rear wall 12 and is spot-welded there'to" at 19 to further increase the rigidity of the housings'tructurel In addition, end caps 25,25 are provided to enclose the endsof the housing. x

Inaccordance with the present invention, the finned tubing? 11 is mounted within the housing 10 by means which facilitates quiet longitudinal moveme'nt'of the tubing due to thermal expansion and contraction and which also removably" secures the front cover 13 to the housing to facilitate acces'stb its interior. To this end, a series of upright webs or elements may be seen in FIG. 2, each web 20 has an upwardlyprojec't ing tooth form 21 supporting thebaffle 1'7- and another upf-f wardly projecting tooth form 22 engaging an inturnedtlip 2''? for supporting the front cover 13. Each web 20 also has a recess 24 extending rearwardly from its front edge 20ato per mit the finned tubing to be disposed centrally in the air space 14..

The tubing is supported in the housing by-a series of resilientclips 30,30 (FIGS. 2, 3, and 4) located adjacent the webs-'- 20, 20. As may be seen in FIG. 4, each-clip 30 has a front- 5 opening U-shaped portion 31 with a series of transverse-bosses" 32,32'projecting radially inward around its inner periphery}? and each clip 30 has a downwardly biased finger 33 which -teb minates in a downturned lip 34 engaging in a mating inturnii lip 35 along the lower margin of the front cover 13. In addition, each clip 30 is secured to its associated web by means of atab 36 and suitable mechanical fasteners. for example bolts or rivets. To permit relatively silent longitudinal expansion and contraction ofthe tubing in the housing, the tubing is mounted by a central clamping action and slidable end mounting which permits the ends thereof to slide in the end clips in response to thermal expansion and contraction. The centralclamping action is provided by a screw 37 which engages between the legs 38 and 39 of the U-shaped portion31 of the central clip 30 to supplement the loose engagement provided by the resilience of the clip. Such supplemental clamping is omitted from the end clips 30. Thus, with this construction, the bosses 32,32 of the clip 30 provide point contact with the finned tubing to minimize heat loss by conduction therefrom while permitting the tubing to be readily removed from the housing by displacing the fingers 33,33 upwardly to remove the front cover 13 and withdrawing the tubing forwardly from the U-shaped portions of the clips.

In the present embodiment of the invention, heat is generated in the finned tubing 11 by electrical energy. To this end, a continuous length of electric resistance wire or element 53 is housed in the tubing and is electrically. insulated therefrom by aluminum or magnesium oxide 54 or the like. The element 53 is supplied with electric current by a pair of lead wires 40,40 connected in series with a conventional normally closed limit switch 41 which has an elongated temperature-sensing probe 42 overlying the finned tubing 11 to sense the temperature thereof and open the switch 41 in response to an upper limit temperature. In the present instance, the probe 42 is protected from damage by a tubular element 43 which encloses-it and which is supported by the webs 20,20. If

desired, the lead wires 40,46) may be wired into a conventional room thermostat to provide accurate local temperature control, and, when so wired, overheating of the element within the finned tubing is prevented by the action of the limit switch 41 should the air passage l4 become inadvertently blocked.

As noted heretofore, the prior methods of affixing fins onto a length of tubing for use in baseboard heaters are undesirable, since loose fitting fins may often result. The presence of loose fins on tubing is even less desirable when the tubing embodies electric-heating elements therein, since hot spot" may occur in the tubing, shortening the life of the heating elements and possibly causing them to burn out. In addition, the cost of materials in fabricating the heater, including the cost of the alloy-heating elements, makes it desirable to provide an effcient electric baseboard heater having a high capacity for heat output per unit of length. Furthermore, in the interest of producing a relatively low-cost heating unit, it is desirable to reduce the costs in mounting the fins onto the elongated tubes.

To this ends, a series of platelike, preferably aluminum, fins 50,50 (FIG. ll) each having a noncircular central aperture 51, is mounted on a thimwalled, preferably aluminum or copper, length of tubing or tubular conductor 52, each aperture having a series of arcuate surfaces 58,58 alternating with a series of inwardly projecting substantially straight surface I portions 59,59. The tubing 52 has a noncircular cross section L normal to its longitudinal axis 52a which is identical to but slightly smaller than the noncircular aperture 51. Similarly to the aperture 51, the cross section of the tubing 52 has a series of outwardly facing arcuate portions 67,67 alternating with outwardly facing straight portions 66,66, the arcuate portions 67,67 each having a uniform radius of curvature measured from the axis 520. Each fin 50 is locked onto the tubing 52 by a series of interference fits 55,55 located at spaced intervals around the inner periphery of its aperture 51, the fins being dead soft" permitting them to be deformed radially outward of the tube. As may be seen in FIGS. 9-42, the inwardly facing straight surfaces 59.59 on the fins are spaced a distance x from 1 the axis 52a and the outwardly facing arcuate surfaces 67,67

on the tubing 52 are spaced the distance r from the axis 52a. In the present invention, the distance x is less than the distance r. Moreover, the inwardly facing arcuate surfaces 58,58 on the fins are spaced from the axis 52a a distance y which is slightly greater than r to provide a small clearance 75 therebctween. The circumferential lengths of the portions 58 and 66 are greater respectively than the lengths of the portions 67 and 59. Thus, with these critical dimensions, the fins may be readily displaced along the length of the conductor tubing 52, but the interference fits 55,55 may be effected by the novel method which will be described hereinafter.

In the present instance, four interference fits are employed, each providing a contact length of B (FIG. 12), where B is substantially equal to one-eighth the circumference of the tub stantially equal to one-half the circumference (piXr) of the tubing. If desired, the length of B may be varied for each interference fit. Thus, for tubing diameters greater than about I inch, five, six or more fits may be employed. In these instances, the total length of contact of the fit would still be approximately one-half the circumference (piXr); however B would be equal to or thereby conforming to the general formula B JI ing and the total contact length is subwhere N is equal to the number of interference fits.

It is to be noted that the heat transfer efficiency of the finned tubing is high even though, as seen in FIGS. 11 and 12, an air space 56 exists on opposite sides of each fit 55. Although there is little heat transfer by conduction between the fins and the tubing across these spaces; nevertheless, heat transfer between the fins and the tubing is not impaired. This is because heat transfer by conduction is increased by the presence of the metal-to-metal bond created, by the interference fits, and because an increased area for conductive heat transfer is provided at each interference fit by a skirt encircling the aperture 51 in each fin. As may be seen in FIG. 7, the skirt 60 projects rearwardly from the plane of the fin and terminates in a peripheral flange 61. The flange 61 provides structural rigidity to the skirt 6%, so that the skirt is deformed and elastically engages the tubing at the interference fits.

The efficiency of the finned tubing of the present invention has been established by tests. Thus, tests on finned tubing hav ing rectangular fins approximately 2" by 2%" and skirt lengths of about V1" mounted on a tube having an outside diameter of about /3" have established that the contact area provided by the skirt 60 in accordance with the above parame ters is sufficient to transfer as much heat from the tubing to the fins by conduction as the fins can dissipate to the air by convection under ambient conditions. Moreover, tests have further established that an increase in heat transfer capacity of about 5-7 percent per lineal foot may be effected by the finned tubing of the present invention over an arrangement where the fins are crimped or swaged onto the tubing. Thus, the heat transfer efficiency of the unit is not impaired by the presence of the air spaces 56 between the fins and the tube.

In order to fabricate the finned tubing at economical production rates, a novel method in accordance with the present invention is employed. To this end, the method comprises the steps of forming a noncircular aperture in each fin, forming an identical but slightly smaller cross-sectional sh ape on a length of tubing, aligning the tubing with the apertures, displacing the tubing longitudinally through the apertures, and turning the fins through a portion of a turn relative to the tubing to lock the fins into position by interference fits between them and the tubing. The steps employed in this method are illustrated schematically in Flt 5--l2.

Referring to FIGS. 5, 5a and 5b, the tubing 52 has a round or circular cross section (FIG. So) when the resistance wires 53,53 and oxide material 54 are mounted therein. The round tube 52 is then advanced axially between a series of opposed rollers 65,65 (FIGS. 5 and 6) which cooperate to form the flat surfaces 66,66 on the tubing, the flat surfaces alternating with the arcuate surfaces 67,67 around the periphery of the tubing to result in the aforementioned noncircular cross section (FIG. 5b). An identical noncircular cross section is formed on the skirt 60 and around the inner periphery of the aperture 51 by suitable metal-forming apparatus. As may be seen in FIG. 8, the fins 50,50 are positioned in a fixture 70 which aligns their apertures with each other, so that when the tubing 52 is aligned with the aperture in the end fin, it may be readily ins erted through the apertures in the other fins. After the fins are properly positioned on the tubing, they are locked in place by rotating them through an angle a (FIG. 11) to deform the skirts and provide the interference fits 55,55. This is preferably done by clamping the ends of the tubing and rotating the fixture 70. In the present instance, angle a is equal to one quarter of a turn or 45; however, its magnitude will vary depending on the diameter of the tubing and the number of interference fits desired, as discussed heretofore. In addition, in the illustrated embodiment, the clearance 75 is about .003" between the apertures of the fins and the tubing when they are aligned to thereby facilitate insertion of the tubing through the apertures (See FIGS. 9 and 10). If desired, a suitable noninsulating lubricant may be applied to the tubing before it is displaced through the apertures to further facilitate insertion of the tubing therethrough.

in view of the foregoing, it should now be apparent that an improved method has now been provided for economically making finned tubing which is efficient in operation, and which has increased heat transfer capacity per unit length. Moreover, novel means has also been provided for mounting finned tubing in baseboard housings to permit the tubing to undergo themtal expansion and contraction with a minimum of noise.

While preferred embodiments of the'present invention have been described in detail, various modifications, alterations or changes may be made without departing from the spirit and scope of the present invention.

' lclaim:

l. A heat exchanger comprising: an elongated conductor having a shaped peripheral surface with a noncircular cross section normal to a central longitudinal axis therethrough, said noncircular cross section providing a series of outwardly projecting portions at spaced intervals about the periphery thereof, a plurality of fins spaced-apart on said conductor, each of said fins having an aperture with a series of inwardly facing surfaces disposed at spaced intervals about the inner periphery thereof and each of said fins being secured to said conductor by a series of interference fits between said inwardly facing surfaces and said outwardly projecting portions of the peripheral surface of said conductor, whereby conductive heat transfer between said conductor and said fins is provided, said spacing between said outwardly projecting portions of said conductor surface being at least as great as the circumferential length of said inwardly facing surfaces and said spacing between said inwardly facing surfaces being at least as great as the circumferential length of said outwardly projecting portions of said conductor.

2. A heat exchanger in accordance with claim 1 wherein said outwardly projecting portions of the peripheral surface on said conductor comprise a series of arcuate portions spaced apart therearound and said inwardly facing surfaces on said fins engage said arcuate surfaces on said conductor to provide the interference fits.

3. A heat exchanger in accordance with claim 2 wherein said shaped peripheral surface on said conductor has a series,

of substantially straight portions disposed intermediate said arcuate portions around its periphery, said straight portions on said arcuate portions around its periphery, said straight portions on said conductor being disposed on opposite sides of each interference fit and in spaced confronting relation with said aperture in each fin.

4. A heat exchanger in accordance with claim 3 wherein each fin has a skirt projecting outwardly from the plane of the fin and surrounding the aperture therein, said skirt defining said inwardly facing surfaces and having a peripheral flange spaced from said plane, said flange reinforcing said skirt so that said inwardly facing surfaces thereon elastically engage said conductor.

- 5. A heat exchanger in accordance with claim 4 wherein the length of each interference fit is substantially equal to where r is the radius of curvature of the arcuate portions of the conductor and N is equal to the number of interference fits.

6. A heat exchanger in accordance with claim 4 wherein the total length of engagement between each fin and said arcuate portions of the conductor is substantially equal to one-half of the circumference of said conductor.

7. A heat exchanger in accordance with claim 4 wherein the peripheral length of each arcuate portion is substantially equal to one-eighth of the circumference of said conductor. 

